The present invention generally relates to sensors for electronic instrumentation and, more particularly, to temperature compensation, linearization, and amplification of the output signal of a transducer.
Transducers are used in a wide variety of engineering applications to convert various physical quantities to an electrical signal. An example of one type of such a transducer is the pressure transducer, which usually includes a pressure sensor and some associated circuitry for producing an electrical output. Pressure transducers may be used for applications as varied as sensing oil pressure in an engine to sensing cabin air pressure in an aircraft. A pressure transducer typically provides an output voltage or current that is a function of pressure applied to the sensor. For example, the output voltage or current may be some specific value at zero pressure and may increase in proportion to the increase in pressure on the sensor. In such a case, the transducer output signal voltage or current is a linear function of the input pressure, and the transducer output signal is said to be linear. Although linearity of the transducer output signal is desirable, it is typically not the case. For most sensors, the specific values of output voltage or current at zero pressure and full-scale pressure will also change with temperature.
Sensors are generally coupled in transducers through at least a rudimentary signal conditioning circuit to electronic instrumentation for use in an application. The conditioning circuit is typically an analog circuit, as opposed to a digital circuit. Due to imperfections in sensor element manufacturing, the sensor signal conditioner must be able to compensate the transducer output signal for the span and offset of the sensor output over the operating temperature range. For example, in the case of the pressure sensor output voltage or current, the sensor signal conditioner must be able to adjust the span—the range of voltage or current output between the highest and lowest output voltage or current—and must also be able to adjust the offset—the particular output voltage or current for a certain value of pressure at the sensor, usually zero or equilibrium pressure. The span and offset adjustments for a conventional analog sensor signal conditioner are usually adjusted using a set of trim pots, or potentiometers, to set trim values, which may be, for example, voltages. In addition to imperfections in sensor element manufacturing, the conditioning circuit itself may contribute to non-linearity of the transducer output signal. Temperature, both ambient temperature and temperature changes caused by power dissipation in the circuit and sensor, may also affect the transducer output signal.
Currently, temperature compensation is performed using either an analog or digital approach. An analog approach adds resistors and thermistors (Positive Temperature Coefficient (PTC) or Negative Temperature Coefficient (NTC)). The analog approach is not very accurate, however, due to the repeatability of thermistors and limited selection in resistor values. Also, the analog approach is very time consuming since it requires several iterations in resistor installation and temperature test. Only linear or second order compensation is feasible in this approach.
With the development of integrated circuits, circuits for sensor signal conditioning have been developed using features such as analog-to-digital conversion and non-volatile memory look-up tables. Digital sensor signal conditioner circuits allow greater flexibility in transducer design and increase transducer accuracy and usability. Digital sensor signal conditioner circuits may take advantage of computer controlled instruments and digital communication with a computer by using a procedure to set certain parameters, such as the trim values described above, in order to provide temperature compensation, linearization, and amplification for the sensor.
Prior art procedures for setting trim values typically start by sending an estimated trim value to the sensor signal conditioner circuit, which may be implemented, for example, in an integrated circuit (IC) chip, such as an application specific integrated circuit (ASIC). The estimated trim value is used with a test input value applied to the transducer at a controlled temperature to produce a readout on a computer controlled instrument. In the pressure sensor example, a test pressure would be applied to the sensor with the sensor at a known temperature, and the readout on a computer-controlled instrument would be checked against a desired value. Based on the readout, the estimated trim value is adjusted and the process is repeated until the readout from the computer controlled instrument falls within a predetermined error limit. Then the same repetitive procedure is done on multiple sensing and multiple temperature ranges. For example, using the pressure sensor, the repetitive procedure would be done at a number of different pressure and temperature combinations.
Continuing to use the pressure sensor as an example, the pressure reading can be derived from the sensor output using following equations:                     Vout        =                ⁢                              Z            ⁡                          (              T              )                                +                                    S              ⁡                              (                T                )                                      *            P                                                  =                ⁢                              (                                          A                0                            +                                                A                  1                                *                T                            +                                                A                  2                                *                                  T                  2                                            +              ⋯                        ⁢                                                  )                    +                                    (                                                B                  0                                +                                                      B                    1                                    *                  T                                +                                                      B                    2                                    *                                      T                    2                                                  +                ⋯                            ⁢                                                          )                        *            P                                                  =                ⁢                              C            0                    +                                    C              1                        *            P                    +                                    C              2                        *                          P              2                                +          ⋯                    where P=pressure in psi;                T=temperature in ° C. or ° F.;        Vout=sensor output, typically measured in milliVolts (mV);        Z(T)=zero offset of sensor, which is a function of temperature;        S(T)=sensitivity of sensor, which is a function of temperature; and        Ai, Bi, Ci=curve-fit coefficients (constants).Temperature compensation is accomplished through the Ai and Bi coefficients, where the Ai coefficients are used for zero offset and the Bi coefficients are used for the sensitivity. Correction for non-linearity is accomplished through the Ci coefficients.        
For example, a second order linearity correction requires solving the following equation:Vout=C0+C1*P+C2*P2.In general a non-linear equation, such as the above, can be approximated using either a polynomial fit or a piecewise-linear fit.
For example, a second order polynomial fit requires measuring sensor output, Vouti, at three different pressure inputs, P1,. P2,. and P3, and substituting the values into the following equations:Vout1=C0+C1*P1+C2*P12Vout2=C0+C1*P2+C2*P22Vout3=C0+C1*P3+C2*P32The Ci coefficients can be calculated by solving these three equations simultaneously.
The solution can also be approximated by two straight lines using a piecewise-linear fit. Using a piecewise-linear fit also requires measuring sensor output, Vout, at three different pressure inputs, P1,. P2,. and P3. Vout can then be approximated using the following equations:Vout=D0+D1*P when P1<P<P2Vout=E0+E1*P when P2<P<P3where the Di and Ei coefficients can be calculated by substituting the values for Vout and P1,. P2,. and P3 and then solving each of the two linear equations. The calculation is much simpler than the calculation for a polynomial fit.
For second order temperature compensation, a polynomial fit may be performed by solving the following equations in a manner similar to that described above. Measurements are made at six temperatures, T1 through T6.Vout1=(A0+A1*T1+A2*T12)+(B0+B1*T1+B2*T12)*PVout2=(A0+A1*T2+A2*T22)+(B0+B1*T2+B2*T22)*PVout3=(A0+A1*T3+A2*T32)+(B0+B1*T3+B2*T32)*PVout4=(A0+A1*T4+A2*T42)+(B0+B1*T4+B2*T42)*PVout5=(A0+A1*T5+A2*T52)+(B0+B1*T5+B2*T52)*PVout6=(A0+A1*T6+A2*T62)+(B0+B1*T6+B2*T62)*P
A piecewise-linear fit may be performed by solving the following equations in a manner similar to that described above. Measurements are made at three temperatures, T1 through T3.Vout=(F0+F1*T)+(G0+G1*T)*P when T1<T<T2Vout=(H0+H1*T)+(I0+I1*T)*P when T2<T<T3The Ai and Bi coefficients can be derived by simultaneously solving the polynomial equations or the Fi, Gi, Hi, and Ii coefficients can be derived by simultaneously solving the two linear equations in a manner similar to that described above.
Companies that design and market digital signal conditioning IC chips use the above approach almost exclusively. The calculations are very extensive and typically require either a computer or a special calibration set up in order to perform the task of setting up and calibrating the IC chip. The calibration set up is very expensive since, for example, it may have to accommodate two-way digital communication between the IC chip and a computer and, also, analog data transmission of multiple sensors.
Prior art procedures, such as that just described, to provide temperature compensation, linearization, and amplification of the output of a transducer using an ASIC may be implemented using a computer. For example, a computer is connected to the ASIC chip using a data communication protocol, such as RS-232. The computer trims the transducer output until the desired output signal is achieved. The computer then sends an adjusted value to the ASIC chip. The ASIC chip takes the adjusted value from the computer, and a digital-to-analog (D/A) circuit converts the adjusted value to alter the output signal. The output is then measured by a digital multi-meter (DMM) and the measured output value is sent to the computer. The computer then makes a further decision on whether the adjusted value is too little or too much. The process is cyclic and can last for a few minutes. Once the computer decides that the adjusted value is close enough, the adjusted value is “burned” in to the EPROM or EEPROM on board the ASIC. This procedure is repeated for a number of different pressure and temperature combinations.
Despite the advantages of using ASIC chips for temperature compensation, linearization and amplification of transducer output, including better sensing accuracy, the current practice does not take full advantage of the cost-saving features of the digital approach because of the extensive hardware and software setup involved in adjusting the output and setting trim values as described above. For example, because every particular combination of sensor and ASIC integrated circuit chip is slightly different, individual trimming of each individual combination of sensor and ASIC conditioning circuit is required during mass production of transducers, i.e., the combination of sensor and ASIC conditioning circuit. In order to achieve individual trimming of each transducer, some form of individual communication is typically setup between a computer, or other means of controlling the trimming process, and each transducer. For example, a computer could assign a different “address” to each individual transducer as part of a communication protocol between the computer and all the transducers. Such communication setup may be complicated and time-consuming, adversely affecting some of the advantages of mass production, such as time and cost efficiency.
As can be seen, there is a need for setting trim values to provide temperature compensation, linearization, and amplification of the output of a transducer, which avoids extensive hardware and software setup. Also, there is a need for setting trim values to provide temperature compensation, linearization, and amplification of the output of a transducer, which can be performed simultaneously on many transducers despite differences between individual components.